It's impossible to buy this application since there is no website to acquire it from. When you are trying to load the website, a blank page is displayed with the name of the domain which means it's probably discontinued without certain belief that will come back ever again. I've also used various websites and database alternatives to look for something you can use instead, but there isn't.
You can use other apps from the same category using the following search results: http://software.informer.com/search/todo+list